Comparison of clozapine and norclozapine concentrations in serum, dipotassium EDTA (K2EDTA) plasma and lithium heparin plasma

Introduction: Plasma and serum are often used interchangeably for therapeutic drug monitoring (TDM). TDM is recommended for the antipsychotic clozapine, although few studies have compared concentrations between plasma collected using different anticoagulants and serum. Therefore, K2EDTA plasma, lithium heparin plasma and serum were compared for the measurement of clozapine and norclozapine.

Methods: Whole blood from patients (N = 30) prescribed clozapine was collected into separate polypropylene blood collection tubes containing K2EDTA, lithium heparin (without a polymer gel) and a serum separator blood collection tube containing silica particles and a polymer gel. Samples were prepared using liquid liquid extraction, and analysed using high pressure liquid chromatography with ultraviolet detection. Imprecision (RSD%) was < 5% at low and high concentrations of clozapine (126, 636 µg/L) and norclozapine (128, 428 µg/L).

Results: Mean clozapine concentrations in K2EDTA plasma, lithium heparin plasma, and serum were 678, 727, and 559 µg/L, respectively. There was no significant difference between K2EDTA plasma and lithium heparin plasma (p = 0.54), and between lithium heparin plasma and serum (p = 0.07). However, serum clozapine was significantly (p = <0.05) lower than K2EDTA plasma. Regarding norclozapine, mean concentrations in K2EDTA plasma, lithium heparin plasma, and serum were 275, 303, and 283 µg/L, respectively. There was no significant difference between K2EDTA plasma and serum, and K2EDTA plasma and lithium heparin plasma. However, serum norclozapine was significantly (p = <0.05) lower than lithium heparin plasma.

Conclusion: Overall clozapine and norclozapine concentrations were significantly lower in serum as compared to either K2EDTA or lithium heparin plasma. The cause of this difference is unclear, although the presence of a gel in the blood collection tube used to collect serum maybe a cause. Caution should be exercised when using serum and plasma interchangeably, and laboratories should investigate any potential differences between matrices using their own methodology.